1. Field of the Invention
The present invention relates to an optical switch, more particularly to a photochromic optical switch which is switchable by a small quantity of light.
2. Description of the Prior Art
There has been known various optical switches of an optical waveguide type. In one of the switches, optical waveguides are formed on an electrooptical crystal such as LiNbO.sub.3. In another of the switches, the optical waveguides and electrodes are formed on a semiconductor substrate such as GaAs, and the refractive index of a part of the optical waveguides is changed by applying a voltage to the part, introducing an electrical current into the part, or the like, to switch an optical path over.
These optical switches of the optical waveguide type are shown in FIG. 11 and are described on page 92 of "The Research And Study Report I" regarding optical function devices, which was published by "The Japanese Electronic Industry Development Association" (hereinafter referred to as "Document 1") in 1988. In particular, an optical switch of an optical directional coupler type, and an optical switch of a total internal reflection type are being widely researched for the purpose of practical use.
In a typical example of the optical switch of the total internal reflection type, an n-type semiconductor is embedded in X-intersecting portions of optical waveguides as shown in FIG. 12, and the refractive index of the intersecting portions is increased through electric field modulation to perform the switching by total internal reflection, as described on page 96 of Document 1.
On the other hand, as an optical switch using a photochromic material, there has been known a conventional optical switch of an on/off type which utilizes the change in the absorbing action of the photochromic material. An optical switch utilizing the change in refractive index under the photochromic effect of the photochromic material is being developed recently. Further a photochromic optical switch using an optical fiber coupler was proposed in "The Summaries C-292 of The Lectures at the Spring Convention" (hereinafter referred to as "Document 2") of "The Institute of Electronics, Information & Communication Engineers" of Japan in 1991 the optical fiber coupler 15 shown in FIG. 13, and FIG. 14 shows a sectional view of the coupling portion of the switch shown in FIG. 13. The switch of FIGS. 13 and 14 includes a switching light source 1, a filter 2, the coupling portion 3 in which optical fibers are optically coupled to each other through fusion, and a clad layer 5 containing photochromic material 14.
Since the switch of the total internal reflection type shown in FIG. 12 and described in Document 1 requires a process of forming cores in a semiconductor substrate made of InP, GaAs or the like where other semiconductors all embedded in the substrate, it takes much time and trouble to manufacture such a switch, and the cost is high. Further, since the semiconductors are high in refractive index, the coupling loss due to boundary reflection is high when they are connected to optical waveguides or optical fibers of quartz glass.
On the other hand, the photochromic optical switch shown in FIG. 13 and described in Document 2 is easy to manufacture, and advantageous to connect to optical fibers. However, since the two optical fibers are coupled with each other through fusion in the coupling portion 3, as shown in FIG. 14, the coupling portion is coated with the clad layer 5. This clad layer is made of a compound which is variable in refractive index and which contains the photochromic material 14. This allows it to utilize the switching effect caused by the change in the refractive index around the coupling portion.
The region of the coupling portion 3 between two optical waveguides permits most of the power of switching light for the switch to pass through. Therefore, since the photochromic material 14 cannot be disposed in the region of the switch which has the optical fibers fused with each other, the switch cannot be sufficiently operated unless the thickness of the variably refractive compound is 10 .mu.m or more at minimum. However, since the photochromic material 14 has a strong absorbing action at, and near, the wavelength of the switching light for photochromic reaction, in principle, the light is less likely to penetrate as deeply into the switch when the concentration of the material or the thickness of the layer 5 is increased to this level. For that reason, the quantity of the switching light needs to be made very large for switching.